1. Field of the Invention
This invention relates generally to magnetic tunnel junctions (MTJs) and particularly to measuring the resistance value of the MTJ.
2. Description of the Prior Art
As already known, the resistance of a magnetic tunnel junction (MTJ) is indicative of its magnetic state and therefore can be used to determine the logical state of a memory device using MTJ. Measuring or detecting the resistance of the MTJ is commonly employed in detecting the data that is stored in the MTJ. More specifically, the resistance of the MTJ is notably different when the stored data in the MTJ is one or zero. This is because the MTJ's magnetic layers are parallel relative to one another when the stored data is zero, and the MTJ's magnetic layers are anti-parallel relative to one another when data is one. The MTJ resistance is an effective metric for detecting the stored data. Resistance of a MTJ with relevant magnetic layers having a parallel orientation is commonly of a lower value, Rlow, as opposed to the resistance of the MTJ with relevant magnetic layers having an anti-parallel orientation, which is typically a notably higher resistance, Rhigh.
Measuring the resistance of a MTJ provides Rlow, Rhigh and tunnel magneto-resistance (TMR, which is (Rhigh−Rlow)/Rlow). This information can be used for MTJ characterization, evaluation, and for debugging of process problems. Currently, some of this information is collected from test chips, which is limited to a small number of MTJs for measurement. In order to collect a vast amount of data, the actual product, typically a chip made of or including a magnetic random access memory (MRAM) array with a multitude of MTJs can be employed. The MRAM array provides an excellent candidate for measuring Rlow, Rhigh. However, currently, there is no reliable structure or manner for using MRAMs to measure Rlow, Rhigh.
What is needed is a MRAM having the capability for reliably measuring the MTJ resistances that can be used for MTJ characterization. This would help the MRAM design to increase reliability of the MRAMS particularly during reading/sensing operations.